When I was looking for mine, I spent two years looking for "the one." Got "the one" and started what was supposed to be a rather superficial interior refurb, and it turned into the dreaded shell-off. The old ice-pick trick can help you identify rot in the floor, but the only way you will know how bad the frame is is by dropping some portion of the belly-pan. For me, I wish I would have bought the first one I saw three years ago. That way, I would be done with my rebuild and camping right now.
My opinion is that buying a trailer that is 40 years old is like buying a car of that age. Even if the thing was kept in a museum, there is going to be lots of stuff that will have dried out, cracked, and need repair. If you find one that was kept in a museum, the price will reflect it.
In my search, I ran across many trailers that had been someone else's project, but they lost their drive to finish it. These were trailers that had a nice fresh floor and a stripped interior. If you can find one like this, then the owner might be able to show you pictures of the frame repairs and floor replacement. This way, you focus on laying out the interior, and you have a manageable project.
Be careful about traveling too far to buy a trailer. The further you drive, the more likely you are to buy the thing, even if it doesn't meet your expectations.